High frequency condenser



Aug. 29, M TE 2,171,219

H IGH FREQUENCY CONDENSER Filed Dec. 30, 1937 INVENTOR.

LOU/5 MALTER ATTORNEY.

Patented Aug. 29, 1939 UNITED STATES PATENT OFFIQE HIGH FREQUENCY CONDENSER Application December 30, 1937, Serial No. 182,403

4. Claims.

My invention relates to condensers, particularly to condensers of low impedance for use in high frequency circuits.

The capacity of a conventional condenser is fixed by the area of the plates, the distance between the plates, and the dielectric constant of the insulation between the plates. Many condensers with the necessary low impedance and high current carrying capacity, for establishing a minimum potential between two points in a radio frequency circuit, are costly and too large for convenient use in some electrical apparatus. The larger types of R. F. condensers, further, have inadmissible losses in the dielectric when operated at ultra-high frequencies.

An object of my invention is to make a condenser for use in high frequency circuits which is small in dimensions for a given capacity, has low leakage losses, and is easy and inexpensive to manufacture.

A further and more specific object of my invention is to make a radio frequency condenser which is small in size, yet has an effective impedance of zero at the operating frequency.

A more complete understanding of my invention may be obtained by referring to the following specification and to the accompanying drawing in which Figure 1 shows diagrammatically a condenser embodying my invention, Figure 2 is a view of a condenser, similar to the coin denser of Figure l, with folded insulated plates, Figure 3 is a View of a multi-leaf condenser embodying my invention, Figures 4 and 5 show my improved condenser constructed with discs, particularly for use in connection with high frequency transmitter tubes, Figure 6 shows my improved condenser built into the end of a concentric transmission line, and in Figure 7 are curves showing the operation of my improved condenser.

The condenser embodying my invention and shown by way of example in Figure 1 comprises plates I and 2 spaced by a dielectric material 3, such as a sheet of mica, with terminal connections 4 and 5 connected in a high frequency circuit between, for example, a point of high frequency potential and a second or grounded point. To permit the easy flow of high frequency current in the circuit the impedance between terminals 4 and 5 must be low with a corresponding high value of effective capacity between plates l and 2. For zero impedance the condenser of conventional construction would necessarily be infinite in size.

' At ordinary radio frequencies up to frequencies corresponding to wavelengths of several meters, the condenser capacities of the conventional condenser vary in direct proportion to the area. of the registering plates, the capacitive reactance varies with the area of the plates as indicated by 5 the curved dotted line in Figure 7. In the conventional condenser the reactance approaches but never reaches Zero regardless of the size of the plates or of the frequency employed. I have found, however, that if the dimensions of the 10 plates are so constructed as to be commensurate with the wavelength of the operating frequency, the direct proportionality between reactive impedance and area no longer hold and that when the greatest dimension of the condenser is about one-tenth of the wavelength the deviation increases with increase of frequency. I have found that as the length of the condenser plates, measured from their terminals to the points on the plates most remote from the terminals, are increased the capacitive reactance between the terminals of the condenser reduces, and at a length equal to one-quarter of a wavelength of the impressed frequency the reactance becomes zero and the effective capacity of the condenser becomes infinite. The relation between the electrical length L of my condenser and the reactance X between its terminals is graphically represented by curve a in Figure 7. If the electrical length of the plates of my condenser is increased to a value greater than a quarter wavelength, the reactance becomes inductive. The reactive impedance of my improved condenser becomes zero at the third quarter wavelength, as indicated by curve b, Figure 7, and at odd numbered quarter wavelengths.

While the precise electrical operation of my improved condenser is not known, it is believed that the elongated plates function much the same as an open ended transmission line in which the current wave distribution is such that the current at the input end of the line is a maximum and in phase opposition to the voltage wave which at the input end is zero.

A condenser constructed according to my invention has many advantages in high frequency circuits because 01" its small size and its low impedance. Whereas the length of condenser plates 5 and 2 is fixed by the wavelength of the operating frequencies, the width and accordingly the area of the plates is limited only by the energy dissipating properties of the plates and their dielectric insulator 3.

Should the length of the condenser plates 1 and 2 in a flat plane as shown in Figure 1 be too great,

they may conveniently be folded with two or more non-inductive convolutions as shown in Figure 2. For minimum impedance between terminals 4 and 5 at the operating frequency, the electrical length of plates I and 2 is adjusted to equal an odd number of quarter wavelengths.

Alternatively the electrical plates or electrodes of my condenser may be made up of a number of leaves, segments or sections interleaved as shown in Figure 3 in which the distance between the terminal and the point most remote from the terminal, measured longitudinally along the surfaces of the leaves, is adjusted to the quarter-wavelength of the operating frequency. To prevent the current from flowing around the edges of the leaves and in effect short-circuiting the high frequency paths along the surfaces of the leaves, the edges of the leaves may be of a dielectric. The leaves may, for example, be made of thin sheets of insulating material such as mica or ceramic material plated with a metal coat of good conducting material or covered with strips of metal foil spaced inwardly from the edges of the leaves.

In the device of Figure 3 the current flows from terminal 5 along the outer surfaces of condenser leaves 6, to the right along the inner surfaces of leaves 6, and to the left along the two sides of leaf 1, the effective end of the transmission line being the left end of leaf 1. Current from terminal 4 follows a similar circuitous path along the surfaces of leaves 8 to a point on the interior of the condenser opposite terminal 4. Because the high frequency current flows in two directions from each terminal, 4 and 5, the condenser of Figure 3 may be considered as two open-ended transmission lines in parallel.

Because of the skin effect in conductors at high frequencies, solid metal leaves may be employed for guiding the current and. for fixing the electrical length of the condenser plates. At frequencies corresponding to several meters the current flows only on the surface of the conductor, and at a frequency of 100 megacycles per second the depth of penetration of high frequency current is less than .001". My improved condenser may, if desired, be circular in construction as shown in Figure 4 with a plurality of metal discs, alternate discs being connected at their centers to one terminal of the condenser and the remaining discs connected attheir outer periphery to the other terminal of the condenser. Discs 9, l and ll are joined at their centers to terminal member or rod l2, and discs |3 and I4 spaced at their centers from rod l2 are joined along their edges to terminal cylinder I5. The thickness of the several discs is greater than the depth of penetration of the applied high frequencies and the junctions between the discs and the terminal members are made impervious to the high frequency current by solder or brazing so that all current flowing into the condenser follows radial paths over the surfaces of the discs. The electrical length of the leaves of the condenser plate made up of the three discs 9, l0 and H is fixed by twice the sum of the radii of the discs plus the distances between the discs. A condenser constructed according to Figure 4 may conveniently be employed as the terminating impedance of a concentric transmission line and by properly adjusting the electrical length of the condenser plates the terminal impedance of the transmission line becomes zero. In the case of the structure of Figure 4 with disc-shaped condenser leaves having axial symmetry and with the leaves perpendicular to the terminal, axis it has been found that the electrical length of the condenser for zero reactance is slightly greater than one-quarter wavelength of the operating frequency. This divergence from the exact odd onequarter wavelength relationship arises from the fact that the static capacity between immediately opposite ring-shaped increments of the leaves increases with the radial distance from the axis.

Figure 5 shows an application of my improved condenser constructed similar to the condenser of Figure 4. In the glass portion N5 of the envelope of a transmitting tube is sealed an annular lead-in ring H for a tube electrode such as the screen grid. In the operation of this tube at wave lengths of the order of five meters, it is important that the screen grid be grounded through a low impedance condenser. The lower the impedance of the grounding connection the lower is the radio frequency voltage on the screen h grid and the more stable is the operation of the tube. Two sets of circular ring-like plates are interleaved with alternate leaves l8 joined solidly to the lead-in ring and the remaining leaves l9 connected along their outer periphery to a grounding shield. The impedance of the connection between the electrode lead-in ring and ground may easily be made zero by adjusting the electrical length of the condenser leaves equal to one or an odd quarter wavelength of the operating frequency.

According to another embodiment of my invention the condenser may be conveniently made for terminating the end of a concentric transmission line as shown in Figure 6. Cylinders or leaves 28, 2| and 22 of one electrical plate of my improved condenser are of different diameters and are coaxially mounted at their ends on a metal disc 23, supported at its center on the end of the inner conductor 22 of the concentric transmission line. Leaves 25 and 26 of the other electrical plate of the condenser are interleaved with leaves 20, 2| and 22 and are coaxially supported and mounted at their ends upon disc 21 closing the end of the outer tube 28 of the concentric transmission line. Rod 29 is screw threaded in the center of disc 21. The leaves and disc are sufficiently thick to prevent the flow of current through the metal. In operation, high frequency current flows. radially outward in all directions from the end of conductor 24 along the outer surface of disc 23, to the right along the outer surface of cylinder 28 around its end and to the left along the inner surface of cylinder 20, hence longitudinally along the outer and inner surfaces of cylinders 2| and 22 to a point opposite the end of rod 29. High frequency current entering the condenser from the tube 28 of the concentric transmission line flows longitudinally along the inner and outer surfaces of cylinders 25 and 2B terminating at the inner end of rod 29. The electrical length of the condenser is the shortest distance measured along the surfaces of the disc 23 and cylinders 2|], 2| and 22 to a point on the inner surface of cylinder 22 opposite the end of rod 29. The electrical length and static capacity of the condenser may conveniently be changed by telescopingly adjusting rod 29 in its screwthreaded connection to disc 21.

A condenser made in accordance with my invention is small in dimensions for a given capacity, has low leakage losses and is easy and inexpensive to manufacture. My improvedcondenser is particularly useful in radio frequency circuits and although small in size maybe constructed according to my invention to have an effective impedance of zero at the operating frequency.

I claim:

1. A condenser for high frequency current comprising two electrical plates capacitively coupled and a di-electric between said plates, a terminal for each plate, each plate comprising a plurality of leaves, each leaf being conductive on its surface for said high frequency current, the leaves of one electrical plate being disposed between the leaves of the other electrical plate, the plates being so constructed aand arranged that the electrical length of the path of said high frequency current along the registering surfaces of said plates is approximately equal to an odd number of quarter wavelengths of said high frequency current.

2. A condenser for high frequency current comprising two condenser electrodes, each electrode consisting of a plurality of coaxial metal cylinders joined at their ends to a disc, the free ends of the cylinders of one electrode being telescoped in interleaved relation with the cylinders of the other electrode, a terminal member for supporting each disc and the cylinders in spaced relation, the electrical length of the registering surfaces of the cylinders for said high frequency current being about equal to an odd number of quar- 30 ter-wavelengths of said high frequency current.

3. A condenser for high frequency current comprising a plurality of spaced discs electrically conductive on their surfaces for said current, coaxially mounted at their centers on a terminal member, a second set of discs electrically conducting on their surfaces and coaxial with the first mentioned discs, the discs of said second set interleaved with the discs of the first mentioned discs and joined along their edges to a terminal cylinder, the electrical length of the registering surfaces of said discs being approximately an odd quarter wavelength of said current.

4. A condenser for high frequency current comprising two electrical plates capacitively coupled, with a dielectric between said plates, a terminal for each plate, each plate comprising a plurality of leaves, each leaf being conductive only on its surface for said current because of its high frequency, the leaves of one electrical plate being disposed between the leaves of the other electrical plate, each plate being so shaped and of such size that the length of the path for said high frequency current along the registering surfaces of said plates from its terminal to the point on the plate most remote from said terminal is approximately equal to an odd number of quarter wavelengths of said high frequency current.

LOUIS MALTER. 

